Large database search using CAM and hash

ABSTRACT

A hash-CAM (H-CAM) which may work with a controller and a memory containing a database of either search values and associate content or associate content by itself The H-CAM includes at least one set of paired hash units and CAM units and at least one logic unit. The CAM units hold values known to cause hash collisions in the respectively paired hash units, and the logic unit prioritizes the hash and CAM unit outputs to a single address value usable to access the memory and obtain a search result at the controller that is not the result of a hash collision. The H-CAM may optionally include a search data storage to store the search values, so that they need not be stored in the memory, and a comparator to determine and handle newly determined hash collisions.

TECHNICAL FIELD

[0001] The present invention relates generally to static information storage and retrieval systems, and more particularly to architectures for searching large databases of stored data using hash algorithms or associative memory, also referred to as content addressable memory or tag memory.

BACKGROUND ART

[0002] Many real world systems require searching information at very high speed; hence hardware based approaches are often employed. An increasingly common example, and the one which will primarily be used herein, is searching in network systems. In information searching there are basically two types of information searching: exact match and partial match searching. The present invention is intended to solve problems in exact match searching. Two approaches widely used today for exact match searching are hash-based searching and content addressable memory or CAM-based searching.

[0003] To compare these two types, for instance, one can consider the problem of designing and searching a database that can support one million entries of 128-bit input data and 32-bit of associated data.

[0004] In a hash based search, the concept is to map the search data, which is often very lengthy as in this 128-bit example, to numbers which are often much smaller, say only 20-bits in length, since only 20 bits are needed to address one million entries. Since this conversion (128-bit to 20-bit) is not a one-to-one mapping, collisions will very likely occur. That is, a hash collision is said to have occurred here when the hash function employed returns the same 20-bit result for two different 128-bit inputs.

[0005] Furthermore, depending on the particular data encountered and the particular hash function employed, more than just two different 128-bit inputs can be mapped to the same 20-bit result number. It therefore is not uncommon for a system to have to be designed to accommodate the fact that three, four or even more different inputs will be mapped by the hash function to the very same output number. There are many different approaches to reducing the impact of hash collisions. One approach is to increase the database size. For instance, consider first the degenerate case, a “1-way-set-associative hash.” Here the 128-bit input search values are mapped to 20-bit values used as address indexes into a memory. The memory needs to be 1M in size, since each 20-bit value needs to map to an actual memory location. This case, of course, does not handle any hash collisions at all. It simply ignores them, and therefore is not very practical. Consider next a “2-way-set-associative hash.” Here one set of collisions can be handled. The memory needs to be 2M in size, since each 20-bit value needs to map to two actual memory locations. Higher-way associative hashes can also be used, but beyond a “4-way-set-associative hash” this approach of simply increasing the database size is typically not practical. Based on the parameters used here, in a 4-way case the memory would need to be 4M in size and would be very poorly utilized.

[0006]FIG. 1 (background art) is a block diagram depicting a search engine 10 using conventional hash-based database searching. A controller 12 includes a hash function 14 which can receive 128-bit input search values and generate 20-bit hash value which is used as an index to address a memory 16.

[0007] If we assume that hash collisions will not happen too often, and use only a 2-way-set-associative hash, the memory 16 needs to be able to store a database having two million entries. This is depicted here as a base region 16 a and a conflicts region 16 b in the memory 16, a total of two regions for a 2-way-set-associative hash. A 21-bit wide address bus 18 is therefore needed to communicate between the controller 12 and the memory 16 (20-bits to address the first one million entries in the base region 16 a, and one additional bit to address the second million entries, in the conflicts region 16 b used to support the one set of potential collision cases). The entries in the memory 16 each require 160 bits, 32 bits for an associate value which is the desired result of the search and 128 bits for the input search value which is intended to produce that particular result.

[0008] The above illustrates a key point in hash-based database searching—both the associate content value and a stored instance of the search value which produces it must be stored in the memory 16 and returned to the controller 12, here via a 160-bit result bus 20, for the controller 12 to perform a comparison to determine if a hash collision has occurred. The controller 12 can only do this if it has both the input and stored search values. If these are the same in a search result obtained from the base region 16 a, the associate value in the search result is valid and the controller 12 can carry on with using it. If these values are different, however, a hash collision has occurred. Then the controller 12 accesses the memory 16 a second time, using the 21^(st) bit to address an entry stored in the conflicts region 16 b. If the input and stored search values are now the same, the controller 12 can again carry on. If these are still different, however, another hash collision has occurred and using only a 2-way-set-associative hash and a two million entry database is not adequate for the task at hand.

[0009] A good hash algorithm is one that produces very few collisions. However, it usually cannot be known how many collisions will actually occur, because the pattern of the input search data is typically not known in advance. If there is more than one collision for a given number, a 2-way-set-associative hash will not be able to handle it. In order to support the database with more confidence, 4-way or more set-associative approach should then be used. When such is used, more memory must be provided.

[0010] The size of the memory depends on two things: the number of entries being supported and the number of ways of set-associativity employed. For example, to support n entries using 4-way-set-associativity, the memory size has to have 4 n entries. For one million entries this means that the memory must be four million entries “deep,” even though only one million of entries will ever potentially be used, i.e., less than 25% of the memory is utilized.

[0011] The number of ways of set-associativity also dictates that more clock cycles will potentially be needed for a search. As noted for the 2-way associative hash in FIG. 1, it will take a maximum of two memory read operations (instead of one) to perform a database search, since one collision may happen during the search. Similarly, for an m-way set-associativity it may take up to m memory read operations to perform a database search.

[0012] It follows that hash-based database searching has substantial disadvantages when one considers the large amount of memory needed and the limited speed of searching possible. For discussion, these can be termed the memory size issue and the search speed issue, and both increase linearly when hash-based searching is used. The memory size issue is largely a matter of cost, and thus may be easily solved if cost is not a serious problem. The search speed issue is more difficult, however, and it may make hash-based searching impractical in many applications.

[0013]FIG. 2 (background art) is a basic block diagram depicting a search engine 50 using conventional CAM-based database searching. Here a controller 52 provides a 128-bit input search value to a CAM 54 that will search its own database (of one million entries) and provide a 20-bit index value to a memory 56, where up to one million 32-bit associate content values may be stored.

[0014] Although other types of memory can theoretically be used in place of the CAM 54, content addressable memory, or associative memory as it is also widely known, is particularly useful. When provided an input search value, a content addressable memory will very rapidly provide as its output the address within it of any match (multiple matches should not occur here unless the CAM 54 is improperly programmed). This index, perhaps with appropriate translation by a suitable logic circuit, can then be used as an address into the memory 56.

[0015] The controller 52 here provides the input search value to the CAM 54 via a 128-bit wide search data bus 58; the CAM 54 provides the address index value to the memory 56 via a 20-bit wide address bus 60, and the memory 56 provides the search result to the controller 52 via a 32-bit wide result bus 62. Since the CAM 54 always provides, if anything, a unique address in the memory 56, only one read operation is required.

[0016] TABLE 1 summarizes, along with aspects of the present invention which are discussed presently, the differences between the prior art hash-based and CAM-based approaches when the controllers 12, 52 are ASIC devices and the memories 16, 56 are RAM. From this a number of respective advantages and disadvantages for these approaches become readily apparent. For instance, the hash-based approach has lower cost and power consumption. The CAM-based approach provides a lower pin count at the ASIC (which is highly desirable), uses less memory (overall), never encounters collisions (no dependency is necessary on the nature of the input data values or on the choice of an algorithm), and has a potentially higher search speed (one which is consistent and known). There are still other advantages and disadvantages, but these are the typical major ones.

[0017] Accordingly, it is desirable to find hardware-based approaches of database searching which do not suffer from the respective disadvantages of the conventional hash-based and CAM-based approaches, and which retain or provide additional advantages for these prior art approaches.

DISCLOSURE OF INVENTION

[0018] Accordingly, it is an object of the present invention to provide components to facilitate the construction of more efficient database search engines.

[0019] Another object of the invention is to provide components to facilitate the construction of database search engines which employ hash-based techniques yet are less restricted by hash collisions.

[0020] And another object of the invention is to provide configurable and cascade able components to facilitate the construction of more adaptable and powerful database search engines.

[0021] Briefly, one preferred embodiment of the present invention is a circuit to search an external memory containing search results, by using a search value received from an external controller. A number of serially connected hash units are provided and are each able to receive an input value and generate a hash value. The first such hash unit connects to an input bus connecting the circuit to the external controller. The first input value thus is the search value, and the last hash value becomes a hash output. A number of CAM units are also provided, equaling the hash units and thus defining paired CAM units and hash units. The CAM units each store a CAM database of instances of the input values known to cause hash collisions in their paired hash units. The CAM units also receive input values common with their paired hash units, match those input values against their CAM database, and provide a CAM output if a match exists. A logic unit receives the hash output and the CAM outputs and creates an address value, where the address value represents an address in the external memory. Finally, the logic unit provides the address value on an output bus connecting the circuit to the external memory.

[0022] Briefly, another preferred embodiment of the present invention is a circuit also able to search an external memory containing search results, by using a search value received from an external controller. A hash unit is provided which is able to generate a hash output based on the search value. The hash unit is connected to an input bus connecting the circuit to the external controller. A CAM unit is provided which is able to store a CAM database of instances of the search values known to cause hash collisions in the hash unit. The CAM unit is also connected to said input bus, and it matches the search value against the CAM database, providing a CAM output if a match exists. A logic unit is further provided. It is able to receive any CAM output provided and create an address value based on that, which it provides on an output bus connecting the circuit to the external memory. The address value represents an address in the external memory. The logic unit also receives the hash output and, if no CAM output is provided, it creates a hash address based on that. The logic unit may also receive a hash pointer value and creates the address value based on that. The logic unit yet further receive a hash hit signal, and responsive to that provide the address value on the output bus. A search data storage stores a number of the hash pointer values, representing potential instances of the hash addresses. The search data storage also stores a number of search data values, representing potential instances of the search values. When the search data storage receives a hash address from the logic unit it retrieves a hash pointer value which it provides to the logic unit. The search data storage also retrieves a search data value based on the hash pointer value. An embedded comparator is connected to the input bus and able to receive the search value. It also receives the search data value from the search data storage, and compares the search value and the search data value to determine whether a match exists. If so, it provides the hash hit signal to the logic unit.

[0023] An advantage of the present invention is that it provides a direct cost savings due to improved memory utilization, and it provides indirect cost savings due to reduced bus widths, reduced pin counts, reduced circuit and component footprints, and reduced power usage and heat dissipation needs.

[0024] Another advantage of the invention is that it is easily implemented within many existing search engine schemes, thus permitting rapid adoption and minimal industry change.

[0025] Another advantage of the invention is that it is essentially plug-in usable in many existing search engine schemes.

[0026] And another advantage of the invention is that its speed equals or betters hash-based prior art, which suffers from poor utilization and limited hash collision handling ability, and the speed of the invention approaches that of the CAM-based prior art, which is expensive.

[0027] These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The purposes and advantages of the present invention will be apparent from the following detailed description in conjunction with the appended tables and figures of drawings in which:

[0029] TABLE 1 summarizes differences between the prior art hash-based and CAM-based approaches and a variation of the embodiment of the present invention depicted in FIG. 8;

[0030] TABLE 2 summarizes inputs and stored values in an example described for the embodiment of the present invention depicted in FIG. 6 and the process for that embodiment depicted in FIG. 7; and

[0031] TABLE 3 provides a comparison of two different implementations of a one million entry 8-way-set-assoicative hash-based system, wherein one uses a typical prior art hash-based approach and the other uses the H-CAM with the indirect hash pointer and a cascading mechanism of the present invention.

[0032]FIG. 1 (background art) is a block diagram depicting a simple hash-based database searching example;

[0033]FIG. 2 (background art) is a block diagram depicting a simple CAM-based database searching example;

[0034]FIG. 3 is a block diagram depicting a search engine according to a hash pointer based embodiment of the present invention;

[0035]FIG. 4 is a block diagram depicting a search engine according to a single-level hash-CAM based embodiment according to the invention;

[0036]FIG. 5 is a block diagram depicting a search engine according to a quad-level hash-CAM based embodiment of the invention;

[0037]FIG. 6 is a block diagram depicting a search engine used in an example of a multi-leveled architecture embodiment of the invention, simplified for use in a detailed example;

[0038]FIG. 7 is a flow chart depicting a process suitable for use with the H-CAM element of the embodiment of the invention shown in FIG. 6;

[0039]FIG. 8 is a block diagram depicting a search engine according to an alternate embodiment of the invention, including a H-CAM element having a comparison section;

[0040]FIG. 9a-b show possible sub-architectures for parts of the H-CAM element of the embodiment shown in FIG. 8, wherein FIG. 9a is a block diagram depicting an architecture for first a logic unit and FIG. 9b is a block diagram depicting an architecture for a search data storage;

[0041]FIG. 10 is a block diagram depicting a search engine according to an alternate embodiment of the invention, including a H-RAM element;

[0042]FIG. 11a-b show possible sub-architectures for parts of the H-RAM element of the embodiment shown in FIG. 10, wherein FIG. 11a is a block diagram depicting an architecture for a first logic unit and FIG. 11b is a block diagram depicting an architecture for a search data storage;

[0043]FIG. 12 is a block diagram depicting a search engine incorporating many of the principals discussed for other embodiments;

[0044]FIG. 13 is a block diagram depicting a search engine having the ability to scale for use in large systems by cascading multiple H-CAM elements;

[0045]FIG. 14 is a block diagram depicting a search engine having the ability to scale for use in large systems by cascading multiple H-RAM elements; and

[0046]FIG. 15 is a block diagram depicting a search engine having the ability to scale for use in large systems by cascading multiple CAM, H-CAM and H-RAM elements.

BEST MODE FOR CARRYING OUT THE INVENTION

[0047] A preferred embodiment of the present invention is a system for large database searches which marries both hash-based and CAM-based advantages without the respective disadvantages. As illustrated in the various drawings herein, and particularly in the views of FIG. 3-6, 8, 10, and 12-15, preferred embodiments of the invention are depicted by the general reference characters 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, and 1100.

A Search Engine Using a Hash Pointer:

[0048]FIG. 3 is a block diagram depicting a search engine 100 according to one preferred embodiment of the present invention. The particularly novel aspect in this embodiment is the use of a hash pointer unit 110, which is interposed between a controller 112 and a memory 114. For an example here, lets say that the controller 112 uses 128-bit wide input search values to search a database of up to one million entries of 32-bit associate content value stored in the memory 114. The controller 112 includes a hash function 116 that generates hash addresses (which potentially may be hash collisions). The controller 112 may be substantially the same as the conventional controller 12 discussed with FIG. 1 (background art). Unlike the scheme used in FIG. 1, however, lets use 4-way set-associativity here. Accordingly, using four way access to up to one million addresses, a 22-bit address bus 118 is needed to connect the controller 112 to the hash pointer unit 110.

[0049] The hash pointer unit 110 includes a 4M×20 memory which may contain programmable unique “pointers” to the one million potential entries in the memory 114. The hash pointer unit 110 is 4M deep to handle 4-way associativity to one million potential entries, but it need be only 20-bits wide to provide pointers able to access the actual one million potential entries in the memory 114. A 20-bit wide pointer bus 120 can therefore be used to connect the hash pointer unit 110 to the memory 114.

[0050] Since hash collisions are possible and must still be checked for, the memory 114 accordingly needs to include entries 160-bits wide (128+32). Individual such entries, or search results are then provided to the controller 112 via a 160-bit wide result bus 122.

[0051] The operation here is similar to that of the prior art search engine 10 in FIG. 1, except that the stored search values and associate content values are accessed “indirectly” through the pointer values stored in the hash pointer unit 110. A 128-bit input search value is used by the hash function 116 to generate a 20-bit hash output, which the controller 112 combines with an offset value to create a 22-bit hash address.

[0052] Any of many different schemes will work to convert the hash value to the hash address. For instance, the controller 112 may do this by prefixing a 2-bit offset value to the hash value. Thus, a “00” prefix can represent that the hash output maps to the first “way” in the 4-way set-association used here, a “01” can represent the second way, and so fourth. On a first try with a particular 128-bit input search value, “00” may be prefixed to the hash value. If the search result this produces has a stored search value that matches the original input search value, there has been no hash collision. In contrast, if these values do not match, a hash collision has occurred. Then “01” may be prefixed to the (same) hash value, creating a new hash address, and the hash pointer produced by the hash pointer unit 110 in response to this may similarly be used to retrieve a different search result. This can continue, handling up to four 128-bit input search values that the hash function maps to the same hash value in this embodiment.

[0053] Summarizing, the search speed issue (as termed in the Background Art section) is not changed. However, the memory size issue is greatly changed. Whereas a 4M×160 memory would be required using a prior art 4-way set-associative hash-based approach, the inventive search engine 100 here requires only a 1M×160 memory and a 4M×20 memory—a substantial overall savings.

A Single Level H-CAM and a Search Engine Employing It

[0054]FIG. 4 is a block diagram depicting a search engine 200 according to another preferred embodiment of the present invention. The particularly novel aspect of this embodiment is a single level H-CAM 210. The single level nature of the H-CAM 210 should not be confused with the set-associative nature of a hash approach. That is, the “level” and “way” are different concepts. The H-CAM 210 in the example here is 1-way-set-associative, but that is coincidental with the use here of only a single level.

[0055] The search engine 200 further includes a controller 212 and a memory 214, which both may be substantially conventional. For instance, the controller 212 may be an application specific integrated circuit (ASIC) and the memory 214 may be an external random access memory (RAM).

[0056] The controller 212 communicates an input search value to the H-CAM 210, via a search data bus 216, and the H-CAM 210 then communicates an address value to the memory 214, via an address bus 218. Responsive to the address value, a result value is communicated back from the memory 214 to the controller 212, via a result bus 220.

[0057] As shown, the architecture of the H-CAM 210 includes a hash unit 222, a CAM unit 224, and a logic unit 226. The input search value is provided, via a first path 228 a, to both the hash unit 222 and the CAM unit 224. The hash unit 222 then provides a hash output on a second path 228 b, and the CAM unit 224 may (if it finds a match) provide a CAM output on a third path 228 c (the address within it of such a match). The logic unit 226 receives the CAM and hash outputs, via the paths 228 b-c, and selects one for use as the address value, which it then provides to the memory 214 via the address bus 218 (effectively a last path 228 d).

[0058] The following is an example pertaining particularly to the search engine 200 depicted in FIG. 4. Lets assume here that the search engine 200 will also use 128-bit wide input search values, to search a database of up to one million entries of 32-bit associate values. The search data bus 216 is 128 bits wide, the address bus 218 is 21 bits wide, and the result bus 220 is 160 bits wide.

[0059] The hash unit 222 employs a hash algorithm to map each 128-bit wide input search value to a “narrow” 20-bit wide hash output value. Each input search value received by the hash unit 222 causes it to provide a respective hash output to the logic unit 226. Of course, the hash output will depend on the hash algorithm actually used.

[0060] The CAM unit 224 in this example is a 100K×128 content addressable memory. If an input search value is received in the CAM unit 224 that matches a stored comparison value (a 128-bit wide copy of a previous search value that caused a hash collision), the address of that match is provided by the CAM unit 224, as a CAM output, to the logic unit 226. In contrast, however, if an input search value is received which does not match any comparison value, no CAM output is provided.

[0061] The logic unit 226 here operates in a straight forward manner. A CAM output has higher priority than a hash output. If the logic unit 226 receives only a hash output, that becomes the basis of the address value. If it receives both a hash output and a CAM output, the CAM output becomes the basis of the address value. This is so because the presence of a CAM output indicates that there is an already known and cataloged hash collision case. Using the hash output for the address value here would cause that collision to occur. Alternately, using the CAM output for the address value instead will avoid that collision. If the logic unit 226 receives only a CAM output it can go ahead and use that as the basis for the address value. However, it may be desirable to treat this as an error condition in many embodiments, since a proper hash algorithm should always produce a hash output.

[0062] If necessary, the logic unit 226 also translates the hash or CAM output into an address value that appropriately maps to the memory 214. For instance, the hash output may (naturally or by translation) map to the low-order one million addresses in the memory 214. If the CAM output also maps to the low-order one-hundred thousand addresses in the memory, it needs to be translated to instead map to high-order addresses in the memory 214.

[0063] The memory 214 here includes a base region 214 a for storing search results that are accessed based on the hash outputs, and a conflicts region 214 b for storing search results that are accessed based on the CAM outputs. The search results in the base region 214 a need to include a 128-bit wide stored search value and a 32-bit associate value, for reasons discussed presently. It then follows, because every possible hash output (defined by a 20-bit index value) must map to a search result, that the base region 214 a in the example here needs to have a size of at least 1M×160.

[0064] The search results stored in the conflicts region 214 b of the memory 214, however, do not have to include any stored search values. The CAM unit 224 already includes full 128-bit wide copies of these same search values it uses to index into the conflicts region 214 b, and storing any stored search values in the conflicts region 214 b is unnecessary and wasteful. The conflicts region 214 b here can therefore simply contain the 32-bit associate values, and thus need only be 100K×32 in size.

[0065] The address bus 218, memory 214, and result bus 220 are stylistically shown in FIG. 4 in a manner to emphasize that the base region 214 a and the conflicts region 214 b may be different in size, and that they may even be distinct physical devices. The result bus 220 needs to be 160 bits wide to carry either size of result value, but a 128-bit field can be empty whenever a result value is taken from the conflicts region 214 b.

[0066] The controller 212 receives a result value from the memory 214, and it may check for a new hash collision. The controller 212 can easily determine if the result value includes a stored search value. If not, the result value will have come from the conflicts region 214 b and the 32-bit associate value in the present result value can be used with confidence that it is uniquely associated with the original input search value. In contrast, for a non-empty stored search value, the result value will have come from the base region 214 a in the memory 214 and a hash collision is still a possibility that needs to be dealt with. The controller 212 can then compare the non-empty stored search value with the original input search value. If these are the same, no hash collision has occurred and the search engine 200 can simply carry on and use the 32-bit associate value in the present result value with confidence. However, if these values differ, a new hash collision has occurred and the associate value received as part of the result value should not be used. When a new hash collision is detected in this manner the search engine 200 specifically the CAM unit 224 and the conflicts region 214 b of the memory 214, may be programmed to avoid it in the future. Theoretically, the search engine 200 here can handle hash collisions up to 100,001 deep in a worst case situation, since the conflicts region 214 b has 100K locations to map collisions to after a first entry in the base region 214 a has been used.

A Multi-level H-CAM and a Search Engine Employing It

[0067]FIG. 5 is a block diagram depicting a search engine 300 according to another preferred embodiment of the present invention. The particularly novel aspect in this embodiment is a quad-level H-CAM 310. A controller 312 and a memory 314 are also provided.

[0068] In a manner much similar to the previous embodiment, the controller 312 here communicates an input search value to the H-CAM 310, via a search data bus 316, and the H-CAM 310 then communicates an address value to the memory 314, via an address bus 318. Responsive to the address value, a search result is then communicated back from the memory 314 to the controller 312 via a result bus 320.

[0069] As shown, the architecture of the H-CAM 310 includes a number of paired hash units (hash units 322 a-d) and CAM units (CAM units 324 a-d), and a single logic unit 326. The input search value is provided to the hash unit 322 a and its paired CAM unit 324 a, via a path 328 a. The hash unit 322 a then provides a first hash output on a path 328 b, which conveys it to the next hash unit 322 a where a second hash output is provided on a path 328 c, and so forth until a last, fourth hash output is provided on a path 328 e. Concurrently, the CAM unit 324 a provides a first CAM output on a path 328 i, a second CAM output is provided on a path 328 h, and so forth until a last, fourth CAM output is provided on a path 328 f. Summarizing, at this point there is a single final hash output on a path 328 e and multiple CAM outputs on paths 328 f-i, and all of these paths 328 e-i lead to the logic unit 326. The logic unit 326 selects a value from those it receives to use as the address value, and provides it to the memory 314 via the address bus 318.

[0070] The following is an example particularly pertaining to the search engine 300 depicted in FIG. 5. Inside the H-CAM 310, the path 328 a is 128 bits wide, as it is essentially the same as the search data bus 316; the path 328 b is desirably smaller, say, 64 bits wide; the path 328 c is desirably still smaller, say, 48 bits wide; the path 328 d is desirably yet smaller, say, 32 bits wide; and the paths 328 e-i are all smaller, specifically all 20 bits wide, as one of their contents is ultimately sent via the 21-bit wide address bus 318 (effectively a last path 328 j).

[0071] The purpose of the multiple hash units 322 a-d here is to reduce the input data space, hence the hash outputs should each desirably be smaller than the prior input. The multiple CAM units 324 a-d store entries known to cause hash collisions at the respectively paired hash units 322 a-d. The underlying rationale of this multi-level architecture is that when a given number of hash collisions are expected a corresponding number of CAM entries will be needed. So, based on probability and knowing that the width of the CAMs shrink at successive levels, using multiple levels of CAM in this manner should reduce the total amount of CAM require to provide for the expected number of hash collisions. Nevertheless, the actual number of hash collisions still depends on the particular pattern of input data encountered.

[0072] The logic unit 326 outputs an address value based on a highest priority scheme. The earlier CAM stages have higher priority over later stages, and ultimately over the hash output. For example, the CAM output of the first CAM unit 324 a has higher priority than the CAM output of the second CAM unit 324 b, and so forth, ending here with the CAM output of the fourth CAM unit 324 d having priority over the hash output of the fourth hash unit 322 d. The logic unit 326 may also, if necessary, translate the outputs to an address value that appropriately maps to the memory 314.

[0073] The memory 314 has a base region 314 a, a first conflicts region 314 b, a second conflicts region 314 c, a third conflicts region 314 d, and a fourth conflicts region 314 e. As was discussed for the “highest” conflicts region 214 b in FIG. 4, the fourth conflicts region 314 e does not need to include room for stored search values, since the first CAM unit 324 a contains a copy of the search value and will always match uniquely.

[0074] The hash output of the fourth hash unit 322 d becomes the basis for address values that map to the base region 314 a; the CAM output of the fourth CAM unit 324 d becomes the basis for address index values that map to the first conflicts region 314 b; the CAM output of the third CAM unit 324 c becomes the basis for address index values that map to the second conflicts region 314 c; and so forth. The conflicts regions 314 b-e thus each store data to handle correspondingly higher level collision cases.

A Different Multi-level H-CAM and Search Engine Employing it, in a Detailed Example

[0075]FIG. 6 is a block diagram depicting a search engine 400 according to the present invention, now used in detailed example. A two-level H-CAM 410 is provided, along with a controller 412, memory 414, search data bus 416, address bus 418, and result bus 420.

[0076] A programming unit 430 is also shown here (and many embodiments of the invention will include programming capability, but is not described in particular detail since its task can be performed in various ways and using various architectures that are all substantially conventional or which should be readily apparent to those skilled in the art in view of the following discussion. The result bus 420 also differs from the result bus 220 (FIG. 4) in that it is bi-directional, and the memory 414 here accordingly accommodates that. Some variations of the programming unit 430 may employ these when programming the memory 414.

[0077] As shown, the architecture of the H-CAM 410 here includes a two hash units (hash units 422 a-b) and two CAM units (CAM units 424 a-b), and a logic unit 426. A path 428 a connects the search data bus 416 to the first hash unit 422 a and the first CAM unit 424 a; a path 428 b connects the first hash unit 422 a to the second hash unit 422 b and the second CAM unit 424 b; a path 428 c connects the second hash unit 422 b to the logic unit 426; a path 428 d connects the second CAM unit 424 b to the logic unit 426; and a path 428 e connects the first CAM unit 424 a to the logic unit 426.

[0078] The memory 414 includes a base region 414 a, a first conflicts region 414 b, and a second conflicts region 414 c. These might, for instance, respectively be 1M×160, 200K×160, and 100K×32 in size.

[0079]FIG. 7 is a flow chart depicting a process 500 suitable for use with the H-CAM 410 of FIG. 6. The process 500 starts with a step 510, where the two CAM units 424 a-b and the memory 414 are empty. In a step 512 an input search value is provided by the controller 412 to the H-CAM 410. In a step 514 the input search value is received by and processed by the first hash unit 422 a to generate a first hash output. In a step 516, the input search value is also received by and processed by the first CAM unit 424 a to create a first CAM output (but only if there is a match). In a step 518 the first hash output is received by and processed by the second hash unit 422 b to generate a second hash output. In a step 520, the first hash output is also received by and processed by the second CAM unit 424 b to create a second CAM output (but, again, only if there is a match).

[0080] Summarizing, up to here the H-CAM 410 has internally produced a final (second) hash output, and possibly also either or both of first and second CAM outputs. Next, in a step 522 these outputs are received by and prioritized by the logic unit 426. The first CAM output receives the highest priority; if it is present it is used for the address value. The second CAM output receives the next priority; if it is present and the first CAM output is not, the second CAM output is used for the address value. The (final) second hash output receives the lowest priority; if it is present and the neither of the CAM outputs is, the second hash output is used for the address value. If necessary, the logic unit 426 also translates the respective output used so that the address value maps to a corresponding area in the memory 414.

[0081] In a step 524 it is determined whether a hit was detected in the first CAM unit 424 a. If so, in a step 526 the first CAM output is used to access the memory 414 and the controller 412 will receive a search result (typically only an associate value) stored in the second conflicts region 414 c. Since the contents of the second conflicts region 414 c are always uniquely associated with possible input search values, the process 500 is essentially finished for the present input search value, and the search engine 400 can carry on with whatever further operations on the search result are desired.

[0082] If a hit was not detected in the first CAM unit 424 a, in a step 528 it is determined whether a hit was detected in the second CAM unit 424 b. If so, in a step 530 the first CAM output is used to access the memory 414 and the controller 412 receives a search result stored in the first conflicts region 414 b.

[0083] There still remains a possibility, however, that the first hash output, upon which the second CAM output is based, is a new hash collision. Therefore, in a step 532 it is determined whether the stored search value in the returned search result is the same as the original input search value. If so, in a step 534 the process 500 is essentially finished for the present input search value, and the search engine 400 can carry on with whatever further operations on the search result are desired.

[0084] If the stored search value in the returned search result is not the same as the original input search value, in a step 536 the first CAM unit 424 a can be programmed with the input search value and the second conflicts region 414 c can be programmed with an associate value for this (all under direction of the programming unit 430). The process 500 is essentially finished for the present input search value.

[0085] If there were no CAM outputs (steps 524, 528), in a step 538 the second hash output is used to access the memory 414 and the controller 412 will receive a search result stored in the base region 414 a.

[0086] In a step 540 the search result is examined to see if it contains any value. If so, there again still remains a possibility that the second hash output is a new hash collision, and in a step 542 the returned search result is compared with the original input search value.

[0087] In a step 544 it is determined whether these values are the same. If this is also so, in a step 546 the process 500 is essentially finished for the present input search value, and the search engine 400 can carry on with whatever further operations on the search result are desired.

[0088] In contrast, if the compared values are different, in a step 548 the second CAM unit 424 b can be programmed with the first hash output and the first conflicts region 414 b can be programmed with a search result (stored search value and associate value, all under direction of the programming unit 430). The process 500 is then essentially finished for the present input search value.

[0089] Lastly, if in step 540 it is determined that the search result is empty, in a step 550 a search result based on the input search value and an associate value for it may be programmed into the base region 414 a of the memory 414, under direction of the programming unit 430. The process 500 is then essentially finished for the present input search value.

[0090] With reference now to FIG. 6-7 and TABLE 2, an example is presented. By definition here the input search values are six-digits long and the associate values are two-digits long (base 10 numbers are also used here to facilitate understanding). The search data bus 416 and the path 428 a are six-digit buses, the path 428 b is a four-digit bus, and the paths 428 c-e are all two-digit buses. The address bus 418 is a three-digit bus because it must carry the two-digit content of the second hash output when there are no hash collisions in the H-CAM 410, and also carry three digits otherwise (discussed presently).

[0091] Accordingly, the format 000:000000,00 can be used to reference the memory 414, wherein the three digits to the left of the colon represent an address index value, the six digits between the colon and comma represent a stored search value, and the two digits to the right of the comma represent an associate value.

[0092] The contents of the first CAM unit 424 a can be represented using the format 000:000000, wherein the three digits to the left of the colon represent an address index value and the six digits to the right of the colon represent a comparison value equaling an input search value known to produce a collision in the first hash unit 422 a. That is, the 6-digit comparison value is what is compared for a match and the 3-digit address value is associatively “stored” with it (typically in CAM, where this address is simply based on the storage location of the comparison value), to be used to avoid the hash collision by accessing an alternate location in the memory 414 (i.e., in the second conflicts region 414 c).

[0093] The contents of the second CAM unit 424 b can be represented using the format 000:0000, wherein the three digits to the left of the colon represent an address value and the four digits to the right of the colon represent a comparison value for a hash output known to produce a collision in the second hash unit 422 b. That is, the 4-digit comparison value is what is compared for a match here and the 3-digit address value is associatively stored with it, to be used to avoid the hash collision here by accessing an alternate location in the memory 414 (i.e., in the first conflicts region 414 b).

[0094] Initially (step 510), the CAM units 424 a-b and the memory 414 have nothing stored in them. Typically all the values will be zero, but other conventions can be used. This state is represented in row 1 of TABLE 2.

[0095] A first search value is then entered, say, 324238 (step 512). The first hash unit 422 a generates a first hash output, say, 7783 (step 514). With nothing stored yet in the first CAM unit 424 a, it will not provide any first CAM output (step 516). The second hash unit 422 b will generate a second hash result, say, 63 (step 518). With nothing stored yet in the second CAM unit 424 b, it will not provide any second CAM output (step 520). The logic unit 426 therefore simply picks the second hash output, 63 (step 522). Using this address value, the memory 414 provides the content of address 63, which is nothing so far (step 538). This state is represented in row 2 of TABLE 2.

[0096] The controller 412 will determine that the first search value (324238) has no entry yet in the memory 414 (step 540). One is therefore created in the base region 414 a using the search value and an associate value for it, say, 17. This entry is programmed into the memory 414 at address 63 (step 550), since that is what the hash units 422 a-b here map the search value of 324238 to, and address 63 is presently available. This state is represented in row 3 of TABLE 2.

[0097] A second search value is entered, say, 578901. The first hash unit 422 a will generate a first hash result, say, 6311. The second hash unit 422 b will generate a second hash result, say, 63 again. With nothing stored yet in the CAM units 424 a-b, they do not create any CAM outputs, and the logic unit 426 picks the second hash output, 63. Using this address value, the memory 414 provides the content of address 63, which is 324238, 17. This state is represented in row 4 of TABLE 2.

[0098] The controller 212 now determines that: the search value (578901) has a corresponding entry in the memory 414; a hash collision has occurred because the search field in the search result is different than the input search value used (324238≠578901)(step 542); and that this is a first level hash collision. An entry in the second CAM unit 424 b is therefore created (step 548) using the first hash output (6311) and an available address for an entry in a conflicts region (here the first conflicts region 414 b) of the memory 414, say, address 200. Furthermore, a new entry is also created in the memory 414, at address 100 in the first conflicts region 414 b based on the search value (578901) and an associate value provided for it, say, 23. This state is represented in row 5 of TABLE 2.

[0099] A third search value is entered, say, 322413. The first hash unit 422 a will generate a first hash output, say, 6311 again, which forces the second hash unit 422 b to generate 63 as the second hash output. The first CAM unit 424 a has no match for 322413, so there is no first CAM output. However, the second CAM unit 424 b has a match for 6311, so it provides the associatively stored address index 100 as the second CAM output (step 520). The logic unit 426 evaluates its inputs and then picks the second CAM output, 100. Using this as the address value, the memory 414 provides the content of address 100, which is 578901, 23.

[0100] The controller 412 now determines that: the search value (322413) has a corresponding entry in the memory 414; a hash collision has occurred because the search field in the search result is different than the input search value used (324238≠322413); this is not a first level hash collision; and that this is a second level hash collision. An entry in the first CAM unit 424 a is therefore created (step 536) using the search value (322413) and an available address for an entry in the conflicts region (here the second conflicts region 414 c) of the memory 414, say, address 200. Furthermore, a new entry is also created in the memory 414, at address 200 in the second conflicts region 314 c and based on the search value (322413) and an associate value provided for it, say, 86. No search value is required in the second conflicts region 414 c the memory 414 because the first CAM unit 424 a already contains it and it will always map uniquely (a dash represents this in TABLE 2). This state is represented in row 7 of TABLE 2.

[0101] A fourth search value is entered, say, 578901 again. The first hash unit 422 a generates a first hash output of 6311, which forces the second hash unit 422 b to generate 63 as the second hash output. The first CAM unit 424 a has no a match for 578901, so there is no first CAM output. The second CAM unit 424 b has a match for 6311, so the second CAM output is the associatively stored address index 100. The logic unit 426 evaluates its inputs and then picks the second CAM result, 100. Using this address value, the memory 414 provides the content of address 100, which is 578901, 23.

[0102] The controller 412 now determines that: the search value (578901) has a corresponding entry in the memory 414; and a new hash collision has not occurred because the search field in the search result is the same as the input search value used (578901=578901). No learning is therefore necessary and the contents of the associate value field, 23, can be used by the search engine 400. This state is represented in row 8 of TABLE 2.

[0103] A fifth search value is entered, say, 322413 again. The first hash unit 422 a will generate a first hash output of 6311, which forces the second hash unit 422 b to generate 63 as the second hash output. The first CAM unit 424 a has a match for 322413, so the first CAM output is the associatively stored address index 200. The second CAM unit 424 b also has a match for 6311, so the second CAM output is the associatively stored address index 100. The logic unit 426 evaluates its inputs and then picks the first CAM result, 200 (since a first CAM output gets highest priority). Using this as the address value, the memory 414 provides the content of address 200, which is -, 86 (recall that there are no stored search values in the highest order conflicts region).

[0104] The controller 412 now determines that: the search value (322413) has a corresponding entry in the memory 414; and a new hash collision has not occurred because the search field in the result is the same as the input search value used (322413=322413). No learning is therefore necessary and the contents of the associate value field, 86, can be used by the search engine 400. This state is represented in row 9 of TABLE 2.

[0105] Finally, a sixth search value is entered, say, 324238 again. The first hash unit 422 a will generate a first hash output of 7783 again, which forces the second hash unit 422 b to generate 63 again as the second hash output. The first CAM unit 424 a has no match for 324238, so there is no first CAM output. The second CAM unit 424 b has no match for 7783, so there also is no second CAM output. The logic unit 426 evaluates its inputs and then picks the second hash output, 63. Using this as the address value, the memory 414 provides the content of address 63, which is 324238, 17.

[0106] The controller 412 now determines that: the search value (324238) has a corresponding entry in the memory 414; and a new hash collision has not occurred because the search field in the search result is the same as the input search value used (324238=324238). No learning is therefore necessary and the contents of the associate value field, 17, can be used by the search engine 400. This state is represented in row 10 of TABLE 2.

A H-CAM Including a Hash Pointer and a Search Engine Employing It

[0107]FIG. 8 is a block diagram depicting a search engine 600 according to another preferred embodiment of the present invention. The particularly novel aspect here is a single level H-CAM 610 which employs a variation of the hash pointer (FIG. 3) to perform internal comparisons for detecting hash collisions. The H-CAM 610 works with a controller 612 and a memory 614. These may be substantially conventional, but are both different in some regards from the controller 212 and memory 214 (FIG. 4). In this embodiment the controller 612 does not need to receive any stored search values back for performing comparisons against original input search values (reducing its pin-count and footprint), and the memory 614 does not need to store the search values (reducing its size).

[0108] As shown in FIG. 8, the architecture of the H-CAM 610 is similar in some respects to the H-CAM 210 of FIG. 4. The H-CAM 610 includes a hash unit 622, a CAM unit 624, a first logic unit 640, and a comparison section 652. The salient difference is the comparison section 652, which includes a search data storage 654, a comparator 656, and a second logic unit 658.

[0109] With reference now also to FIG. 9a-b, these depict possible internal architectures for the first logic unit 640 and the search data storage 654. FIG. 9a is a block diagram depicting one architecture for the first logic unit 640, wherein it receives the hash output, the CAM output, and also a CAM hit signal (the mere presence of a CAM output denotes a hit, of course, but using a separate signal provides some speed and logic design benefits). The first logic unit 640 connects to the search data storage 654 and provides it with a hash address based on the hash output.

[0110]FIG. 9b is a block diagram depicting an architecture for the search data storage 654, wherein it is broken into two memory blocks: a hash pointer memory 660 and a search data memory 662. Conceptually the hash pointer memory 660 works much like the hash pointer unit 110 discussed with reference to FIG. 3. The hash pointer memory 660 here receives the hash address and provides a pointer value. It thus has a size based on the number of ways of set-associativity used. The pointer value is then used as an address input to the search data memory 662, which here contains the stored search values that are used to compare with the original input search values to detect hash collisions. The hash pointer memory 660 typically will be much deeper (m>n in FIG. 9b) than the search data memory 662, because its purpose is to increase the utilization of the search data memory 662. Most of the hash pointer memory 660 will be empty but that is acceptable, since it is used to reduce the collision probability in the search data memory 662. With a proper memory depth ratio between the hash pointer memory 660 and the search data memory 662, the amount of the search data memory 662 required can be reduced.

[0111] As can be seen in FIG. 9a, the comparison section 652 provides the pointer value to the first logic unit 640 and it is what is ultimately used in this embodiment to access the base region of the memory 614. The second logic unit 658 passes the pointer value of a search to the first logic unit 640 (the mere presence of a value denotes a hit but using a separate signal, as is depicted, provides some speed and design benefits). Thus, if 4-way set-associativity is used the first logic unit 640 can provide four hash addresses to the comparison section 652 and it will provide up to one pointer value back. The controller 612 can then also be signaled, via a hit line 664, that the search result on the result bus 620 is a valid one.

[0112] With reference briefly again to TABLE 1, it also summarizes the difference between the prior art hash-based and CAM-based approaches and a simple embodiment of the invention depicted in FIG. 8, and FIG. 9a-b. The specifications are essentially the same as for the prior art examples, except that a size of 128×100K is chosen for the CAM unit 624 based on an assumption that not more than 10% of the total database size need be devoted to handling hash collisions. As can be seen, the H-CAM 610 has the same speed as the prior art CAM-based approach. It has roughly the same total bus pin-count at the controller 612 as the prior art CAM-based approach (the hit line 664 may require as little as a single-pin) and it has a substantially lower pin-count than the prior art hash-based approach. This item bears some further comment. In the H-CAM 610 a result bus 620 can be as narrow as 32 bits wide, since it carries only the associate value. In contrast, the result busses 20, 220 have to be 160 bits wide to carry both the stored search value and associate value.

[0113] In TABLE 1 the H-CAM 610 is shown having roughly the same cost and power consumption as the prior art hash-based approach, but this is somewhat misleading. The H-CAM 610 here employs only 2-way set-associativity. If a higher degree of multi-way set-associativity is used additional cost and power savings can be had, providing the present invention with even greater advantage over the hash-based and CAM-based prior art.

[0114] For an example lets now consider a 4-way set-associative case wherein 128-bit wide search values are used to search a database of one million entries. The memory 614 needs to be 1M×32 in size, to handle only the one million desired entries (100% utilization is possible because using the pointer values are used). Furthermore, since a high percentage of hash collisions can be handled by the use of multi-way set-associativity, the CAM unit 624 can be small, say, only 32K×128 in size. It should particularly be noted that increasing the degree of multi-way set-associativity in this manner permits the CAM unit 624 to become decreasingly small, almost trivially small. This is very beneficial if actual content associative type memory is used (typically necessary where high speed is desired). Such true CAM requires more die area and consumes more power than most other types of memory, and usually is much more expensive. Reducing the amount of CAM used is therefore desirable.

[0115] Continuing with the example, the hash pointer memory 660 needs to be 4M×20 and the search data memory 662 needs to be 1M×128. The hash addresses need to be 22-bits wide, to address the four million locations in the hash pointer memory 660. The pointer values are 20-bits wide, since they only need to address the entries (associate content) in the memory 614 and to also address the one million entries (stored search values) in the search data memory 662. The total memory required in the example above is equivalent to 4M×61 (1M×32+32K×128+4M×20+1M×128), which is a marked reduction from the 4M×160 a conventional 4-way set associative approach would require.

[0116] There are many different ways to implement the comparison section 652, and the architecture of these (and the hash algorithm) play an important role in overall efficiency. If designed correctly, this approach is able to reduce the total amount of memory (CAM and other) required. As can be seen in TABLE 1, this can produce a substantial cost savings. Basically, two things can be improved. The first is the handling of hash collisions by using a multi-way set associative hash, and the other is to reduce the depth of the search data storage 654, because the search values are usually very wide.

A H-RAM and a Search Engine Employing It

[0117]FIG. 10 is a block diagram depicting a search engine 700 according to another preferred embodiment of the present invention. The particularly novel aspect here is a H-RAM 710 which also employs a variation of the hash pointer (FIG. 3) to perform internal comparisons for detecting hash collisions. The H-RAM 710 works with a controller 712 and a memory 714. As was the case for the H-CAM 610 of FIG. 8, these may be substantially conventional. The controller 712 here also does not need to receive any stored search values back for performing comparisons (reducing its pin-count and footprint), and the memory 714 here also does not need to store the search values (reducing its size).

[0118] As shown in FIG. 10, the architecture of the H-RAM 710 is notable in that it does not employ a CAM. The underlying rationale here is that if a sufficiently high degree of multi-way set associativity is provided, CAM is not needed to address hash collisions.

[0119] The H-RAM 710 includes a hash unit 722, a first logic unit 740, and a comparison section 752. The comparison section 752 includes a search data storage 754, a comparator 756, and a second logic unit 758.

[0120] With reference now also to FIG. 11a-b, FIG. 11a is a block diagram depicting one possible internal architecture for the first logic unit 740 and FIG. 11b is a block diagram depicting one possible internal architecture for the search data storage 754. Turning first to FIG. 11 a, the first logic unit 740 here is simpler than the first logic unit 640 and in FIG. 9a. It converts the (n-bit) hash output into (m-bit) hash addresses, and it performs any desired translation of the (n-bit) indirect hash pointer into the address value for use with the memory 714.

[0121] As can be seen by comparing FIG. 11b with FIG. 9b, the search data storage 754 and the search data storage 654, and for that matter the comparison section 752 and the comparison section 652, may be essentially the same.

A Multi-level H-CAM with Routing Logics, and a Search Engine Employing It

[0122]FIG. 12 is a block diagram depicting a search engine 800 according to another preferred embodiment of the present invention, an embodiment incorporating many of the principals so far discussed into a H-CAM 810. A controller 812 and memory 814 (potentially the same as those used in the embodiments of FIG. 8 and 10) are present again and key points to note are in the architecture of the H-CAM 810.

[0123] The H-CAM 810 here includes hash blocks 862, a CAM block 864, a comparison section 866, and a main logic unit 868. The hash blocks 862 each include a paired input logic sub-unit 870 and hash unit 872. The CAM block 864 includes an input logic sub-unit 874, a number of CAM units 876 (typically equal in number to the hash blocks 862), and an output logic sub-unit 878. The comparison section 866 includes a search data storage 880, a comparator 882, and an output logic sub-unit 884.

[0124] The input logic sub-units 870 of the hash blocks 862 route data off of a common input path 886 into the individual hash block 862 or onward to the CAM block 864, or route the output data from a prior hash block 862 in similar manner. The input logic sub-unit 874 of the CAM block 864 routes data into the CAM units 876, and the output logic sub-unit 878 combines and routes data from the CAM units 876. The output logic sub-unit 884 of the comparison section 866 combines and routes data from the comparison section 866. The data received from the last hash block 862, the output logic sub-unit 878 of the CAM block 864, and the output logic sub-unit 884 of the comparison section 866 is used by the main logic unit 868, which outputs information to the comparison section 866, to the memory 814, and to the controller 812, via a hit line 888.

[0125] The H-CAM 810 can be user configured to work as a 4-level unit, much in the manner described for the H-CAM 310 in FIG. 5; or it can be configured to work as a 2-level unit, much in the manner described for the H-CAM 410 in FIG. 6; or it can be configured to work as a 1-level unit, much in the manner described for the H-CAM 210 in FIG. 4. In particular, it can be configured to work as a 1-level unit similar to the H-CAM 610 and which operates on very large data. The hash blocks 862 can be configured to each handle part of a wide input on the common input path 886, in parallel, or they can be operated in serial fashion as has essentially been described elsewhere herein. The CAM block 864 can handle either case, with its input logic sub-unit 874 and its output logic sub-unit 878 configurable to use the CAM units 876 as one effective large CAM or as smaller amounts of CAM. The comparison section 866 and its output logic sub-unit 884 are similarly configurable to work with the configuration of the other components.

Cascading in a Search Engine, Shown with H-CAMS

[0126]FIG. 13 is a block diagram depicting a search engine 900 according to another preferred embodiment of the present invention, an embodiment employing the ability to scale for large needs by cascading multiple H-CAMs 910 to form a bigger overall system. If the internal architecture of the H-CAMs 910 generally resembles that of the H-CAMs 210, 310, 410, and an expansion bus 922 is added, a controller 912, memory 914, search data bus 916, address bus 918, and result bus 920 may be used which are essentially similar to those used there with. Alternately, if the internal architecture of the H-CAMs 910 resembles that of the H-CAMs 610, 810, and the expansion bus 922 is added, the controller 912, memory 914, result bus 920, and a hit line 924 may be used which are essentially similar to those used there with. The memory 914 may accordingly store the search data values and the associate content values or just the associate content values.

[0127] These architectures combines all the benefits of hash algorithm and CAM to provide an efficient and cost effective way to support searching large databases. The search engine 900 can support very large database searching without using large amounts of memory to accommodate possible hash collisions, yet it can achieve the same search performance as prior art CAM-based approaches without the attendant high cost and power consumption. Although this approach cannot guarantee a specific capacity to avoid hash collisions, similar to prior art hash-based approaches, with the proper hash algorithm and number of ways of set-associativity this variation of the invention can nonetheless achieve very high utilization. This coincides well with a key goal of the present invention, to increase the degree or number of ways of set-associativity without increasing the memory speed or the bandwidth requirement.

[0128] A cascaded approach makes use of the power of distributed processing. If each H-CAM supports an n-way-set-associative hash, the entire system would be able to support (n x number of H-CAM)-way-set associative hash. Multiple H-CAMs, such as the H-CAM 610 shown in FIG. 8, can be cascaded together, such as through the expansion bus 922 as shown in FIG. 13. If n=2 and five H-CAMs are cascaded together, the system can employ a 10-way-set-associative hash and the hash pointer memory and the search data memory within each H-CAM need to only support two reads per input search value, instead of the ten that would be needed in a typical prior art 10-way-set-associative hash approach.

[0129] TABLE 3 provides a comparison of two different implementations of a one million entry 8-way-set-assoicative hash-based system. One uses a typical prior art hash-based approach, while the other uses the H-CAM with the indirect hash pointer and a cascading mechanism. If each H-CAM supports a 2-way-set-associative hash, four are needed to implement this. Each H-CAM will have 256K entries in the search data memory and 512K pointers in the hash pointer memory. Further assuming each search data entry is 128-bits wide, it can be seen that the inventive approach uses only about one-sixth of the memory and requires only one-fourth of the memory bandwidth.

Cascading in a Search Engine, Shown with H-RAMS

[0130]FIG. 14 is a block diagram depicting a search engine 1000 according to another preferred embodiment of the present invention, an embodiment employing the ability to scale for large needs by cascading multiple H-RAMs 1010. In structure the search engine 1000 here much resembles the search engine 900 of FIG. 13, a controller 1012, memory 1014, search data bus 1016, address bus 1018, and result bus 1020 are provided, and an expansion bus 1022 is used to interconnect the H-RAMs 1010.

Cascading in a Search Engine, Shown with Cams, H-CAMS and H-RAMS in Combination

[0131]FIG. 15 is a block diagram depicting a search engine 1100 according to another preferred embodiment of the present invention, an embodiment employing the ability to scale for large needs by cascading multiple of CAMs 1110 a, H-CAMs 1110 b, and H-RAMs 1110 c, with a controller 1112, memory 1114, search data bus 1116, address bus 1118, and result bus 1120 and all interconnected by an expansion bus 1122. The search engine 1100 thus is particularly powerful and configurable to various applications, including searching very large and highly specialized databases.

Summary

[0132] A lot has been presented herein. The prior art hash approach was discussed, including multi-way set-associativity (see e.g., FIG. 1 (background art)), and the prior art CAM approach was also discussed (see e.g., FIG. 2 (background art)). A novel hash pointer approach was then discussed (see e.g., FIG. 3), including how it provides the benefits of multi-way set-associativity without the poor memory utilization that is characteristic of the prior art.

[0133] A novel H-CAM approach was next discussed, with coverage of it including how it provides many of the benefits of the respective hash and CAM approaches, but without many of the respective disadvantages. FIG. 4 depicted a single level H-CAM variation, and FIG. 5 depicted a multi level H-CAM variation. Both of these examples use 1-way set-associativity, for simplicity, but using multi-way set-associativity is a straight forward extension. FIG. 6 covered programming, which is essentially also straight forward and possible with generally conventional techniques, and FIG. 6 and the flow chart of FIG. 7 were used in a detailed example.

[0134] A novel “local” comparison approach for moving the task of comparison into the H-CAM was discussed (see e.g., FIG. 8 and 9 a-b), employing a variation of the hash pointer approach (FIG. 3). As is the case with just the hash pointer approach, this local comparison approach provides particular benefit when used with multi-way set-associativity.

[0135] A novel H-RAM approach was then discussed, one which uses a variation of the local comparison approach (see e.g., FIG. 10 and 11 a-b). Enhancing the multi level H-CAM by using internal routing logics was also discussed, with optional local comparison included (see e.g., FIG. 12). Finally, cascading multiple H-CAMs, cascading multiple H-RAMs, and cascading multiple CAMs, H-CAMs, and H-RAMs together in combination were shown (FIG. 13-15).

[0136] Accordingly, while various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Industrial Applicability

[0137] The present search engines 100, 200, 300, 400, 600, 700, 800, 900, 1000, 1100, and the individual H-CAMs 210, 310, 410, 610, and 810 and the individual H-RAM 710 are well suited for application in the searching of databases of associate content. The search engines, H-CAMs, and H-RAM can search large databases, particularly when variations such as the configurable embodiment depicted in FIG. 12 or the cascade able embodiments depicted in FIG. 13-15 are used.

[0138] The invention can substantially reduce the memory needed for database storage. The embodiment depicted in FIG. 3 shows how the invention permits memory size to not be based on the degree of multi-way set-associativity used. The embodiments depicted in FIG. 4-6 show how database memory may also be reduced by not storing instances of the search values in a highest level conflicts region in the main memory, and the embodiments depicted in FIG. 8-12 show how database memory may be reduced to store only associate content, and then with a very high degree of utilization.

[0139] The invention can also substantially reduce the time needed for conducting database searches. While the embodiments depicted in FIG. 3-6 show that the invention can at least equal the search speed of hash-based prior art approaches, the more sophisticated embodiments depicted in FIG. 8-11 show how the invention can better the search speed of hash-based prior art approaches and approach the speed of CAM-based prior art approaches.

[0140] The invention can also substantially reduce the overall search engine and database storage system cost. This is achieved directly by increasing memory utilization, and thus reducing the need for memory. Cost reduction is also further achieved indirectly, by reducing bus widths, pin counts, circuit foot print, power consumption (and dissipating heat), etc.

[0141] For the above, and other, reasons, it is expected that the present invention will have widespread industrial applicability. Therefore, it is expected that the commercial utility of the present invention will be extensive and long lasting.

Large Database Search Using CAM and HASH Inventors: CHENG, Paul; CHOW, Nelson L.; and CHIEN, Fangli Atty. ref.: 60409.300901

[0142] THIS CORRESPONDENCE CHART IS FOR EASE OF UNDERSTANDING AND INFORMATIONAL PURPOSES ONLY, AND DOES NOT FORM A PART OF THE FORMAL PATENT APPLICATION.  10 search engine  12 controller  14 hash function  16 memory  16a base region  16b conflicts region  18 address bus  20 result bus  50 search engine  52 controller  54 CAM  56 memory  58 search data bus  60 address bus  62 result bus  100 search engine  110 hash pointer unit  112 controller  114 memory  116 hash function  118 address bus  120 pointer bus  122 result bus  200 search engine  210 H-CAM  212 controller  214 memory  214a base region  214b conflicts region  216 search data bus  218 address bus  220 result bus  222 hash unit  224 CAM unit  226 logic unit  228a-d paths  300 search engine  310 H-CAM  312 controller  314 memory  314a base region  314b-e conflicts regions  316 search data bus  318 address bus  320 result bus  322a-d hash units  324a-d CAM units  326 logic unit  328a-j paths  400 search engine  410 H-CAM  412 controller  414 memory  414a base region  414b-c conflicts regions  416 search data bus  418 address bus  420 result bus  422a-b hash units  424a-b CAM units  426 logic unit  428a-e paths  430 programming unit  500 process  510-550 steps  600 search engine  610 H-CAM  612 controller  614 memory  620 result bus  622 hash unit  624 CAM unit  640 logic unit  652 comparison section  654 search data storage  656 comparator  658 logic unit  660 hash pointer memory  662 search data memory  664 hit line  700 search engine  710 H-RAM  712 controller  714 memory  722 hash unit  740 logic unit  752 comparison section  754 search data storage  756 comparator  758 logic unit  766 comparison section  800 search engine  810 H-CAM  812 controller  814 memory  862 hash blocks  864 CAM block  866 comparison section  868 main logic unit  870 input logic sub-units  872 hash unit  874 input logic sub-unit  876 CAM units  878 output logic sub-unit  880 search data storage  882 comparator  884 output logic sub-unit  886 input path  888 hit line  900 search engine  910 H-CAMs  912 controller  914 memory  916 search data bus  918 address bus  920 result bus  922 expansion bus  924 hit line 1000 search engine 1010 H-RAMs 1012 controller 1014 memory 1016 search data bus 1018 address bus 1020 result bus 1022 expansion bus 1100 search engine 1110a CAMs 1110b H-CAMs 1110c H-RAMs 1112 controller 1114 memory 1116 search data bus 1118 address bus 1120 result bus 1122 expansion bus 

What is claimed is:
 1. A circuit to search an external memory containing search results based on a search value received from an external controller, comprising: a hash unit able to generate a hash output based on the search value, wherein said hash unit is operationally connected to an input bus connecting the circuit to the external controller; a CAM unit able to store a CAM database of possible instances of the search values known to cause hash collisions in said hash unit and able to match the search value against said CAM database such that a CAM output is provided if a match exists, wherein said CAM unit is also operationally connected to said input bus; a logic unit able to: receive any said CAM output provided and to create an address value based there on and to provide that said address value on an output bus connecting the circuit to the external memory, wherein said address value represents an address in the external memory; and otherwise receive said hash output and to create one or more hash addresses based there on, to receive a hash pointer value and create said address value based there on, and to receive a hash hit signal and responsive there to provide said address value on said output bus; a search data storage able to: store a plurality of said hash pointer values, wherein said hash pointer values represent potential instances of said hash addresses; store a plurality of search data values, wherein said search data values represent potential instances of the search values; receive a said hash address from said logic unit and based there on retrieve a said hash pointer value and provide it to said logic unit; and retrieve a said search data value based on said hash pointer value; and a comparator operationally connected to said input bus and able to receive the search value there from, to receive said search data value from said search data storage, to compare the search value and said search data value to determine whether a match exists and, if a said match exists, to provide said hash hit signal to said logic unit, thereby permitting the external memory to not store any instances of the search values.
 2. The circuit of claim 1, wherein said logic unit is further able to generate a search hit signal based on said hash hit signal and whether a said match exists in said CAM unit, and to provide said search hit signal to the external controller via a hit line, thereby confirming that a current instance of the search result.
 3. The circuit of claim 1, wherein said hash unit is programmable to employ different hash algorithms.
 4. The circuit of claim 3, wherein said hash unit includes a plurality of pre-programmed hash algorithms, thereby permitting selectively employing a particular said hash algorithm.
 5. The circuit of claim 1, wherein said CAM unit is programmable with new entries for said CAM database, from the external controller, thereby permitting the external controller to program the circuit to avoid new hash collisions.
 6. A circuit for searching an external memory containing search results based on a search value received from an external controller, comprising: hash means for generating a hash output based on the search value, wherein said hash means is also for operationally connecting to input bus means for connecting the circuit to the external controller; CAM means for storing a CAM database of possible instances of the search values known to cause hash collisions in said hash means and for matching the search value against said CAM database such that a CAM output is provided if a match exists, wherein said CAM means is also for operationally connecting to said input bus means; logic means for: receiving any said CAM output provided and creating an address value based there on and for providing that said address value on output bus means for operationally connecting the circuit to the external memory, wherein said address value represents an address in the external memory; and otherwise receiving said hash output and creating one or more hash addresses based there on, for receiving a hash pointer value and creating said address value based there on, and for receiving a hash hit signal and responsive there to providing said address value on said output bus means; search data storage means for: storing a plurality of said hash pointer values, wherein said hash pointer values represent potential instances of said hash addresses; storing a plurality of search data values, wherein said search data values represent potential instances of the search values; receiving a said hash address from said logic means and based there on retrieving a said hash pointer value and providing it to said logic means; and retrieving a said search data value based on said hash pointer value; and comparator means for operationally connecting to said input bus means and for receiving the search value there from, for receiving said search data value from said search data storage means, for comparing the search value and said search data value to determine whether a match exists and, if a said match exists, for providing said hash hit signal to said logic means, thereby permitting the external memory to not store any instances of the search values.
 7. The circuit of claim 6, wherein said logic means is further for generating a search hit signal based on said hash hit signal and whether a said match exists in said CAM means, and for providing said search hit signal to the external controller via a hit line, thereby confirming that a current instance of the search result.
 8. The circuit of claim 6, wherein said hash means is programmable to employ different hash algorithms.
 9. The circuit of claim 8, wherein said hash means includes a plurality of pre-programmed hash algorithms, thereby permitting selectively employing a particular said hash algorithm.
 10. The circuit of claim 6, wherein said controller means is further for programming said CAM means with new entries in said CAM database, thereby permitting programming to avoid newly determined hash collisions.
 11. A method for searching an external memory containing search results by using a search value received from an external controller, the method comprising the steps of: (a) generating a hash output from the search value, wherein said hash value is smaller in size than the search value; (b) comparing the search value against a CAM database of pre-stored instances of the search value known to cause hash collisions, wherein a CAM output is created if a match exists; (c) creating an address value based on said CAM output if a said match exists and otherwise creating said address value based on said hash output, wherein said address value represents an address in the external memory; and (d) providing said address value to the external memory, thereby permitting finding the search results in the external memory based on the search values.
 12. A circuit to search an external memory containing search results based on a search value received from an external controller, comprising: a plurality of serially connected hash units each able to receive an input value and generate a hash value based there on, wherein said input value includes all or part of the search value or a said hash value of a prior said hash unit, and wherein the last said hash value is a hash output; a plurality of CAM units equaling said hash units, whereby each respective said CAM unit is definable as having a paired said hash unit; said CAM units able to store a CAM database of instances of said input values known to cause hash collisions in its paired said hash unit; said CAM units further able to receive a said input value common with its paired said hash unit, to match said input value against its said CAM database, and to provide a CAM output if a match exists; a logic unit able to receive said hash output and said CAM outputs and create an address value based there on, wherein said address value represents an address in the external memory; and said logic unit further able to provide said address value on an output bus connecting the circuit to the external memory, thereby permitting finding the search results in the external memory.
 13. The circuit of claim 12, wherein said hash unit is programmable to employ different hash algorithms.
 14. The circuit of claim 13, wherein said hash unit includes a plurality of pre-programmed hash algorithms, thereby permitting selectively employing a particular said hash algorithm.
 15. The circuit of claim 12, wherein said CAM unit is programmable with new entries for said CAM database, from the external controller, thereby permitting the external controller to program the circuit to avoid new hash collisions.
 16. The circuit of claim 12, further comprising: a plurality of hash input logics, one per said hash unit, selectively able to route said input values to their respective said hash units and to said CAM units; a CAM input logic able to selectively route said input values to said plurality of CAM units; and a CAM output logic able to selectively combine said CAM outputs, thereby permitting configurable application of said pluralities of said hash units and said CAM units.
 17. The circuit of claim 12, wherein said logic unit is further able to create one or more hash addresses based on said hash output and an offset value, and to additionally permit creating said address value based on a pointer value and a hash hit signal, and the circuit further comprising: a search data storage able to store a plurality of hash pointer values and a plurality of search data values, wherein said hash pointer values represent potential instances of said hash outputs and said search data values represent potential instances of said search values; said search data storage further able to receive said hash address from said logic unit, to retrieve a said hash pointer value based on said hash address, to provide said hash pointer value to said logic unit as said pointer value, and to retrieve a said search data value based on said hash pointer value; and a comparator operationally connected to said input bus, said comparator able to receive the search value from said input bus, to receive said search data value from said search data storage, to compare the search value and said search data value to determine whether a match exists, and, if a said match exists, to provide said hash hit signal to said logic unit, thereby permitting the external memory to not store any instances of the search values.
 18. The circuit of claim 17, wherein said logic unit is further able to generate a search hit signal based on said hash hit signal and whether a said match exists in said CAM unit, and to provide said search hit signal to the external controller via a hit line, thereby confirming that a current instance of the search result.
 19. The circuit of claim 17, further comprising: a plurality of hash input logics, one per said hash unit, selectively able to route said input values to their respective said hash units and to said CAM units; a CAM input logic able to selectively route said input values to said plurality of CAM units; and a CAM output logic able to selectively combine said CAM outputs, thereby permitting configurable application of said pluralities of said hash units and said CAM units.
 20. A circuit for searching an external memory containing search results based on a search value received from an external controller, comprising: a plurality of serially connected hash means for each receiving an input value and generating a hash value based there on, wherein the first said hash means operationally connects to input bus means for connecting the circuit to the external controller and wherein said input value includes all or part of the search value or a said hash value of a prior said hash unit, and wherein the last said hash value is a hash output; a plurality of CAM means, equaling said hash means, whereby each respective said CAM means is definable as having a paired said hash mean; said CAM means for storing a CAM database of instances of said input values known to cause hash collisions in its paired said hash means; said CAM means further for receiving a said input value common with its paired said hash means, for matching said input value against its said CAM database, and for providing a CAM output if a match exists; logic means for receiving said hash output and said CAM outputs and for creating an address value based there on, wherein said address value represents an address in the external memory, and said logic means further for providing said address value on output bus means for connecting the circuit to the external memory, thereby permitting finding the search results in the external memory.
 21. The circuit of claim 20, wherein said hash means is programmable to employ different hash algorithms.
 22. The circuit of claim 21, wherein said hash means includes a plurality of pre-programmed hash algorithms, thereby permitting selectively employing a particular said hash algorithm.
 23. The circuit of claim 20, wherein said controller means is further for programming said CAM means with new entries in said CAM database, thereby permitting programming to avoid newly determined hash collisions.
 24. The circuit of claim 20, further comprising: a plurality of hash input logic means, one per said hash means, for selectively routing said input values to their respective said hash means and to said CAM means; CAM input logic means for selectively routing said input values to said plurality of CAM means; and CAM output logic means for selectively combining said CAM outputs, thereby permitting configurable application of said pluralities of said hash means and said CAM means.
 25. The circuit of claim 20, wherein said logic means is further for creating one or more hash addresses based on said hash output and an offset value, and for additionally permitting creating said address value based on a pointer value and a hash hit signal, and the circuit further comprising: search data storage means for storing a plurality of hash pointer values and a plurality of search data values, wherein said hash pointer values represent potential instances of said hash outputs and said search data values represent potential instances of said search values; said search data storage means further for receiving said hash address from said logic means, for retrieving a said hash pointer value based on said hash address, for providing said hash pointer value to said logic means as said pointer value, and for retrieving a said search data value based on said hash pointer value; and said comparator means for operationally connecting to said input bus means, for receiving the search value from said input bus means, for receiving said search data value from said search data storage means, for comparing the search value and said search data value to determine whether a match exists, and, if a said match exists, for providing said hash hit signal to said logic means, thereby permitting the external memory to not store any instances of the search values.
 26. The circuit of claim 25, wherein said logic means is further for generating a search hit signal based on said hash hit signal and whether a said match exists in said CAM means, and to provide said search hit signal to the external controller via a hit line, thereby confirming that a current instance of the search result.
 27. The circuit of claim 25, further comprising: a plurality of hash input logic means, one per said hash means, for selectively routing said input values to their respective said hash means and to said CAM means; CAM input logic means for selectively routing said input values to said plurality of CAM means; and CAM output logic means for selectively combining said CAM outputs, thereby permitting configurable application of said pluralities of said hash means and said CAM means.
 28. A method for searching an external memory containing search results based on a search value received from an external controller, the method comprising the steps of: (a) generating a plurality of hash outputs based on respective input values having decreasing order, wherein said input values include all or part of the search value or a said input value of just higher order; (b) comparing said respective input values against a like number of respective CAM databases such that respective CAM outputs are created if respective matches exist, wherein said respective CAM databases include pre-stored instances of said input values known to cause hash collisions; (c) creating an address value based on said CAM output produced responsive to the highest order said input value producing a said match, if any, and otherwise creating said address value based on the last said hash output, wherein said address value represents an address in the external memory; and (d) providing said address value to the external memory, thereby permitting finding the search results in the external memory based on the search values. 